1. Field of the Invention
The present invention relates generally to an enhanced joint for connecting carbon members, such as graphite electrodes and graphite pins, with at least one carbon member having asymmetrical properties. More particularly, the invention addresses enhanced joints for graphite pins and electrodes with at least one having a cross section with an asymmetrical coefficient of thermal expansion (CTE).
2. Description of Related Art
Carbon electrodes are used in electrothermal furnaces to melt metals and other ingredients used to form metal alloys. (As used herein, the term carbon electrodes includes graphite electrodes.) Generally, the electrodes used in steel furnaces each consist of electrode columns, that is, a series of individual electrodes joined to form a single column. In this way, as electrodes are depleted during the thermal process, replacement electrodes can be joined to the column to maintain the length of the column extending into the furnace. These electrodes are joined into columns via a connecting pin that functions to join the ends of adjoining electrodes. Conventionally, electrodes are joined into columns via a pin (sometimes referred to as a nipple) that functions to join the ends of adjoining electrodes. Typically, the pin takes the form of opposed male threaded sections, with at least one end of each of the electrodes comprising female threaded sections capable of mating with a male threaded section of the pin. Thus, when each of the opposing male threaded sections of a pin are threaded into female threaded sections in the ends of two electrodes, those electrodes become joined into an electrode column. Commonly, the joined ends of the adjoining electrodes, and the pin therebetween, are referred to in the art as a joint.
Alternatively, the electrodes can be formed with a male threaded protrusion or tang machined into one end and a female threaded socket machined into the other end, such that the electrodes can be joined by threading the male tang of one electrode into the female socket of a second electrode, and thus form an electrode column. The joined ends of two adjoining electrodes in such an embodiment is referred to in the art as a male-female joint.
Carbon electrodes and pins may be fabricated by combining calcined petroleum coke and coal-tar pitch binder into a stock blend. In this multi-step process, the calcined petroleum coke is first crushed, sized and milled into a finely defined powder. Generally, particles up to about 25 millimeters (mm) in average diameter are employed in the blend. The particulate fraction preferable includes coke powder filler having a small particle size. Other additives that may be incorporated into the small particle size filler include iron oxides to inhibit puffing (caused by release of sulfur from its bond with carbon inside the coke particles), coke powder and oils or other lubricants to facilitate extrusion of the blend.
The stock blend is heated to the softening temperature of the pitch and is form pressed to create a “green” stock body such as an electrode or pin. For green electrode production, a continuously operating extruding press may be use to form a cylindrical rod known as a “green” electrode. For pin production, the green pin body is formed by die extrusion or by molding in a forming mold to form a “green pinstock”.
The green stock body is heated in a furnace to carbonize the pitch so as to give the body permanency of form and higher mechanical strength. Depending upon the size of the electrodes or pins and upon the specific manufacturer's process, this “baking” step requires the green electrodes or pinstock to be heat treated at a temperature of between about 700° C. and about 1100° C. To avoid oxidation, the green stock body is baked in the relative absence of air. The temperature of the body is raised at a constant rate to the final baking temperature. For electrode or pin production, the green stock body is maintained at the final baking temperature for between 1 week and 2 weeks, depending upon the size of the electrode.
After cooling and cleaning, the baked electrode or pin may be impregnated one or more times with coal tar or petroleum pitch, or other types of pitches known in the industry, to deposit additional pitch coke in any open pores of the electrode or the pin. Each impregnation is then followed by an additional baking step, including cooling and cleaning. The time and temperature for each re-baking step may vary, depending upon the particular manufacturer's process. Additives may be incorporated into the pitch to improve specific properties of the graphite electrode or pin. Each such densification step (i.e. each additional impregnation and re-baking cycle) generally increases the density of the stock material and provides for a higher mechanical strength. Typically, forming each electrode or pin includes at least one densification step. Many such articles require several separate densification steps before the desired density is achieved.
After densification, the electrode or pin, referred to at this stage as a carbonized body, is then graphitized. Graphitization is by heat treatment at a final temperature of between about 1500° C. to about 3400° C. for a time sufficient to cause the carbon atoms in the calcined coke and pitch coke binder to transform from a poorly ordered state into the crystalline structure of graphite. At these high temperatures, elements other than carbon are volatilized and escape as vapors. Carbonized bodies formed in the above manner have generally symmetric cross sectional CTE's.
Carbonized bodies can alternatively be formed by the resistive heating of a stock blend of coke, pitch and, optionally, carbon fibers, or other suitable mixture of carbon filler, reinforcement and matrix materials. Preferably, the stock blend includes raw coke, high melting point pitch and carbon fibers derived from pitch. Optionally, the stock blend may also include calcinated coke, graphite, carbon fibers, coal tar pitch, petroleum pitch, or coking catalysts such as sulfur. As desired, additives may be added to improve the processing characteristics of the blend or to improve the physical characteristics of the graphite electrode or pin. Such additives may be added during mixing or after forming the stock blend. During the process, resistance heating is accompanied by the application of mechanical pressure (this combination is referred to as “hot pressing”) to increase the density and carbonization of the blend. The resulting carbonized body or “preform” is preferably subjected to graphitization after hot-pressing by heating the preform to a final temperature of between about 1500° C. to about 3400° C. to remove remaining non-carbon components and form a material which is almost exclusively graphite. Optionally, after hot-pressing, the preform electrode or pin may be subjected to one or more densification steps employing a carbonizable pitch to further increase the density of the preform prior to the graphitization step. Forming the carbonized bodies through the hot-pressing step results in the carbonized bodies having asymmetrical properties. In this method of preparation, the cross sectional CTE of the resulting carbon body is asymmetric.
After graphitization is completed, the electrode or pin can be cut to size and then machined or otherwise formed into its final configuration. Given its nature, graphite permits machining to a high degree of tolerance, thus permitting a strong connection between pin and electrode in a joint system or between electrode and electrode in a male-female joint system. (As used herein, the term joint includes both a joint system between a pin and an electrode and a male-female joint system between two electrodes.) Machining the graphitized electrode removes only a small fraction of the overall mass of the electrode, while machining the graphitized pin typically removes up to about 40% or more of the mass of the pin. Thus, the material yield is only about 60% for manufacture of connecting pins.
Carbon members having generally symmetric CTE's across their cross sectional dimensions have joints with substantially circular cross sections. As previously described, these joints can be composed of male tangs from graphite pins or graphite electrodes and female sockets from graphite electrodes. Correspondingly, the male tangs and female sockets composing these joints also have substantially circular cross sections. Since the cross sections of the male tangs and the female sockets have generally symmetric CTE's, the stresses induced in the joint by thermal expansion are fairly uniform across the joint interface, the interface between the male tang and female socket.
The stresses caused by thermal expansion are fairly uniform because the thermal expansion across the cross sections of both carbon members occurs at similar rates and in similar directions. As a result of the male tang and female socket both having substantially circular cross sections, the gap around the joint interface is uniform. Since the cross sectional thermal expansion of the carbon members is generally symmetric, this uniform gap allows the male tang and female socket to expand or reduce during thermal cycles without causing disproportionate stresses around the joint interface.
During exposure to heat, the gap around the joint interface reduces with only slight, if any, variation since the thermal expansion of the two carbon members is symmetric. Because of the uniform gap around the joint interface and the carbon member's generally symmetric cross sectional CTE's, the structural integrity of the joint is maintained as the carbon members are exposed to elevated temperatures as seen in an electrothermal furnace.
Joining a carbon member having an asymmetrical CTE across its cross sectional dimension and a carbon member having a generally symmetric CTE across its cross sectional dimension can pose some challenges. As the carbon members are exposed to heat, the differences in CTE's would cause dissimilar rates of thermal expansion across the cross sections of the carbon members. If the cross sections of the male tang and female socket of the carbon members to be joined were both substantially circular, the differing cross sectional CTE's would expand at different rates and induce stress in the joint.
These stresses may arise because the substantially circular cross sections do not allow much variation of the gap around the joint interface to accommodate the differing rates of expansion. If a uniform gap was left around the joint interface, some areas of the gap around the joint interface may be reduced by the differing rates of thermal expansion while in other areas the gap around the joint interface may not be reduced as much. This varying reduction or expansion of gaps around the joint interface occurs because at least one of the carbon members has an asymmetrical CTE across its cross sectional dimension and therefore one dimension of the cross section of the carbon member will expand more than the other dimension.
With no variable gap around the joint interface to compensate for the increased expansion in that one dimension, destructive stresses in that dimension could possibly arise. These destructive stresses could result in a weakening or possible failure of the joint.